The Dulalian Empire wasn't actually an empire until Royal Dulalia received the Gravion Process. Royal Dulalia, based on Down Under, is still alive to this day. The empire can't have had such a history from -400 years ago.

We can infer that the kingdom, eventually empire, DID NOT EXIST until Royal Dulalia found the Down Under turning point. Only after discovering that did he start to build buildings around it, which eventually became his kingdom. Only after THAT did he get the Gravion Process from Harv Gretborn (going a step further, you can infer that he got the Gravion Process to further protect the Down Under ortho) and only THEN did he become an Empire. The amount of ancient history you've written isn't plausible, since the inferents suggest the current Empire is less than 100 years old (assuming estimated lifespan of Ghyllians).